An Early Pleistocene Mg/Ca-18O Record from the Gulf of Mexico: Evaluating Ice Sheet Size and Pacing in the 41-Kyr World

PUBLICATIONS Paleoceanography RESEARCH ARTICLE An early Pleistocene Mg/Ca-δ18O record from the Gulf 10.1002/2016PA002956 of Mexico: Evaluating ice sheet size and pacing Key Points: in the 41-kyr world • Six ice sheet meltwater events identified in Gulf of Mexico seawater Jeremy D. Shakun1, Maureen E. Raymo2, and David W. Lea3 δ18O record from 2.55-1.70 Ma • Events are typically long, occur late in 1Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, USA, 2Lamont-Doherty Earth δ18 benthic O deglaciations, and line 3 up with summer insolation Observatory, Columbia University, Palisades, NY, USA, Department of Earth Science, University of California, Santa Barbara, • This challenges view of early CA, USA Pleistocene marine δ18O as simply recording obliquity-driven Northern Hemisphere ice volume Abstract Early Pleistocene glacial cycles in marine δ18O exhibit strong obliquity pacing, but there is a perplexing lack of precession variability despite its important influence on summer insolation intensity – the Supporting Information: presumed forcing of ice sheet growth and decay according to the Milankovitch hypothesis. This puzzle has • Supporting Information S1 been explained in two ways: Northern Hemisphere ice sheets instead respond to insolation integrated • Supporting Information S2 over the summer, which is mostly controlled by obliquity, or anti-phased precession-driven variability in ice δ18 Correspondence to: volume between the hemispheres cancels out in global O, leaving the in-phase obliquity signal to J. D. Shakun, dominate. We evaluated these ideas by reconstructing Laurentide Ice Sheet (LIS) meltwater discharge to the [email protected] 18 18 Gulf of Mexico from 2.55-1.70 Ma using foraminiferal Mg/Ca and δ O. Our δ Osw record displays six prominent anomalies, which likely reflect meltwater pulses, and they have several remarkable characteristics: Citation: (1) their presence suggests that the LIS expanded into the mid-latitudes numerous times; (2) they tend to Shakun, J. D., M. E. Raymo, and D. W. Lea occur or extend into interglacials in benthic δ18O; (3) they generally correlate with summer insolation (2016), An early Pleistocene Mg/Ca-δ18O record from the Gulf of Mexico: intensity better than integrated insolation forcing; and (4) they are perhaps smaller in amplitude but longer in Evaluating ice sheet size and pacing in duration than their late Pleistocene counterparts, suggesting comparable total meltwater fluxes. Overall, the 41-kyr world, Paleoceanography, 31, these observations suggest that the LIS was large, sensitive to precession, and decoupled from marine δ18O 1011–1027, doi:10.1002/2016PA002956. numerous times during the early Pleistocene – observations difficult to reconcile with a straightforward δ18 Received 25 MAR 2016 interpretation of the early Pleistocene marine O record as a proxy for Northern Hemisphere ice sheet size Accepted 5 JUL 2016 driven by obliquity forcing at high latitudes. Accepted article online 11 JUL 2016 Published online 25 JUL 2016 1. Introduction The Milankovitch hypothesis holds that ice sheets are sensitive to the intensity of summer insolation, which depends on both the tilt of the earth – which varies with the 41-kyr obliquity cycle – and the seasonal distance to the sun – which varies with the 23-kyr precession cycle. The Milankovitch model has had consid- erable success in explaining late Pleistocene ice volume variations of the past one million years, cycles that are concentrated at eccentricity, precession and obliquity frequencies as well as their multiples [Huybers, 2011; Imbrie and Imbrie, 1980; Raymo, 1997]. Nonetheless, the marine δ18O record suggests that the immedi- ately preceding glacial cycles of the late Pliocene-early Pleistocene (3–1 Ma) occurred at the almost purely 41-kyr pacing (Figure 1b) [Huybers, 2007; Pisias and Moore, 1981; Raymo and Nisancioglu, 2003; Ruddiman et al., 1989]. This 41-kyr world is difficult to reconcile with the Milankovitch hypothesis – why is precession variability absent in the early Pleistocene if summer insolation intensity controls ice sheet mass balance? Two hypotheses have been suggested to rectify the apparent conflict between the ice volume changes pre- dicted by Milankovitch forcing and those actually observed in the marine δ18O record. The Integrated Insolation hypothesis points out that the most intense summers are also the shortest, since the Earth orbits faster when closer to the sun [Huybers, 2006]. Since these competing precession-driven effects, intensity ver- sus duration, nearly cancel out when integrated over the course of the summer, one might not expect to see a strong precession signal in ice volume variability. The Antiphase hypothesis instead argues that ice sheets are driven by both obliquity and precession (as expressed in summer insolation intensity), but while obliquity is in phase between the hemispheres (i.e., increased axial tilt causes stronger summers in both hemispheres), precession forcing is anti-phased (i.e., when one hemisphere’s summer occurs closest to the sun, the other’s ©2016. American Geophysical Union. summer occurs farthest from the sun six months later) [Raymo et al., 2006]. Therefore, if a record of global ice 18 All Rights Reserved. volume, such as marine δ O or sea level, was recording ice volume changes in both hemispheres, it would SHAKUN ET AL. EARLY PLEISTOCENE MELTWATER EVENTS 1011 Paleoceanography 10.1002/2016PA002956 only capture the in-phase behavior at the 41-kyr obliquity period, while antiphased precession variability would largely cancel out. The key to distinguishing between these hypotheses is a reconstruction that isolates Laurentide Ice Sheet (LIS) variability independent of, but co- registered with, the marine δ18O record, to determine if ice sheet ablation was driven by obliquity alone or both obliquity and precession. Here we present records of southern LIS Figure 1. Ocean and land records of Pleistocene ice sheets. (a) G. ruber δ18O meltwater to the Gulf of Mexico from Site 625 in the Gulf of Mexico [Joyce et al., 1990]. Joyce et al. [1993] (GOM) based on planktic foraminif- interpreted negative excursions beyond the range they considered attribu- eral Mg/Ca-δ18O and benthic δ18Oat ‰ table to global glacial-interglacial cycles (dashed lines; 1.3 in the early Ocean Drilling Program (ODP) Site Pleistocene, 2‰ in the late Pleistocene) to reflect input of isotopically depleted Laurentide meltwater via the Mississippi River. (b) The LR04 benthic 625 from 2.55-1.70 Ma, an interval δ18O stack [Lisiecki and Raymo, 2005]. (c) 26Al-10Be ages of Laurentide Ice that features prominent 41-kyr glacial Sheet tills in Missouri at 39°N [Balco and Rovey, 2010]. cycles in the global benthic LR04 stack [Lisiecki and Raymo, 2005]. 2. Background Nearly every numerical ice sheet model that has been used to study the Plio-Pleistocene predicts strong precession-driven ice volume variability, in keeping with the Milankovitch hypothesis [Abe-Ouchi et al., 2013; Berger et al., 1999; Clark and Pollard, 1998; Nisancioglu, 2004]. A notable exception is the work of Huybers and Tziperman [2008], who were able to simulate 41-kyr glacial cycles if two conditions were met: the ablation zone was north of ~60°N, where obliquity forcing dominates, and the ablation season was long enough for summer duration to offset summer intensity. Unfortunately, the typical size of the early Pleistocene ice sheets is rather unclear from the geologic record. Taken at face value, the marine δ18O record suggests that early Pleistocene ice sheets were less voluminous than their late Pleistocene counterparts [Lisiecki and Raymo, 2005]; however, mid-continent tills as far south as Kansas and Missouri indicate that the Laurentide Ice Sheet (LIS) reached a maximum extent at that time comparable to that observed in the late Pleistocene (Figure 1c, 2) [Balco and Rovey, 2010; Roy et al., 2004]. The Regolith Hypothesis of Clark and Pollard [1998] reconciles these two observations by invoking extensive early Pleistocene ice sheets with a low profile geometry, which resulted from rapid ice flow over a thick bed of deformable regolith. They further propose that this thick regolith bed slowly eroded away, eventually resulting in ice sheets that were more sluggish, thicker, and less responsive to insolation forcing, leading to the longer period variability observed in the late Pleistocene. Alternatively, the Antiphase hypothesis can accommodate larger ice sheets whose full signal is not recorded in proxies such as δ18O that integrate an out-of-phase signal from both poles. Ultimately, the physical evidence for large ice sheets, upon which the Regolith Hypothesis is predicated, consists of perhaps only two tills, and the interpretation of this evidence remains controversial (Figure 1c) [Balco and Rovey, 2010; Roy et al., 2004]. It is thus unclear if the LIS routinely advanced as far south as the central United States during the early Pleistocene and responded to the precession forcing that would dominate at these latitudes. If so, this pat- tern would strengthen the Regolith and the Antiphase Hypotheses [Raymo et al., 2006], and open up the possibility that the early Pleistocene ice sheets were as voluminous as their late Pleistocene counterparts, but masked in the global δ18O record by hemispherically antiphased precession variability. If, on the other hand, the LIS rarely advanced into the contiguous US and responded only to high latitude obliquity, these two hypotheses would be commensurately weakened and a more straightforward interpretation of the δ18O record, in line with the Integrated Insolation hypothesis, would be implied [Huybers, 2006]. What is needed to address these issues is an early Pleistocene record of variability of the southern margin of the LIS. SHAKUN ET AL. EARLY PLEISTOCENE MELTWATER EVENTS 1012 Paleoceanography 10.1002/2016PA002956 Figure 2. Map of North America showing Laurentide Ice Sheet extent during the Last Glacial Maximum (dark blue) and at its Pleistocene maximum (light blue), as well as the modeled Mississippi cryohydrological basin midway through the last deglaciation (red dashed line, 14.5 ka basin extent from ICE-5G/VM2 model [Wickert et al., 2013]).

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